In Ulothrix/Spirogyra reduction division (meiosis) occurs at the time of zygospore formation. Plant body of Ulothrix and Spirogyra, is gametophytic (haploid), they produce zoogametes (n) which fuses to form zygosporic (2n) diploid, which is a resting spore. Onset of favourable condition zygospore undergoes reductional division, or meiosis to produce zoo- meiospores.